A patent document 1 discloses a technology regarding a recording or reproducing method of stably performing focus control (or focus process, focus automatic control, focus servo, focusing process) and tracking control (or tracking process, tracking automatic control, tracking servo) when recording or reproduction is performed at high speed with respect to a recording medium, such as a DVD. Specifically, firstly, the property of the recording medium is detected from a high-frequency component included in a focus error signal, which is used for the focus process, or a tracking error signal, which is used for the tracking process. The “property” in the present invention may mean (i) a physical and mechanical property caused by the shape, structure, or the like of the recording medium, such as eccentric acceleration and runout acceleration, or (ii) a chemical or natural property corresponding to the state of a recording surface, such as scratches, dusts, finger prints, and grease. Incidentally, one of the reasons that the property is generated is a change in shape of a groove. The fact that an area with the groove partially changed is generated, as described above, is generally caused by that the shape of a stampa is not properly transferred if an injection molding device is suddenly cooled in a manufacturing process. Then, if the state that the acceleration exceeds a predetermined threshold value is continued at predetermined frequency, the rotational speed (and furthermore, linear recording speed) of the recording medium is reduced, and thus the focus process and the tracking process are stably performed.
More specifically, as one specific example of the tracking process, as disclosed in a non-patent document 1, there is invented a three-beam tracking process in a DPP (Differential push-pull) method. The “three-beam tracking process” in the present invention is a technology in which laser light emitted from an optical pickup is made enter diffraction grating, to thereby generate (i) a main beam, which is mainly used for recording data (or information), and (ii) two sub beams, which are mainly used for the tracking process, and in which the two sub beams are used to perform the tracking process. More specifically, for example, the spots of the sub beams are shifted by half of a track pitch, compared with the spot of the main beam, on the recording surface of a single layer type optical disc. The reflected light of the two sub beams are received by a photodetector or the like which is divided into two. The tracking process is performed by detecting a difference in the amount of light (or phase) which is received in each division of the two-divided photodetector or the like.
On the other hand, even a recording apparatus, such as a general disc drive, is provided with a sensor for detecting oscillation or vibration, and if the detected oscillation amount becomes greater than a predetermined amount, the rotational speed is controlled to be reduced.
Patent document 1: Japanese Patent Application Laid Open NO. 2004-62945
Patent document 2: Japanese Patent Application Laid Open No. 2003-312146
Patent document 3: Japanese Patent Application Laid Open NO. 2002-373419
Non-Patent document 1: “optical head for magneto-optical disk”, Electronic Materials, Kogyo Chosakai Publishing, published on Jul. 1, 1988, vol. 27, no. 7, p 73-74